Investigating Density
Grade 9
Lesson Overview: The students will build their powers of observation to make predictions and form hypotheses, test hypothesis, and generate new questions because of discrepent events following testing.

Learning Outcomes: The students will learn the difference between observation and inference; check the inferences through generating new questions and answering those new questions through experimentation.

Student's Prior Knowledge of Experience: The Scientific Method, Volume, Mass

Purpose: Applying the scientific method to investigate density; This lesson leads into learning the formula for and calculating density; Predicting whether objects will sink or float.

Materials Needed:
Teacher Part 1: Time Required 35-45 minutes

• 2 clear glass 250 or 500 mL beakers
• Ice cubes
• 200 mL isopropyl alcohol
• 200 mL water
• Overhead Projector or dry erase board

Student, Part 1:

• Science Journal
• Writing utensil
• Powers of Observation and curiosity

Teacher, Part 2: Time Required 25-35 minutes

• 200 mL isopropyl alcohol
• 200 mL water
• Yellow and blue food coloring
• Ice cubes
• Plastic wrap
• 500 mL beaker

Student, Part 2:

• Science Journal
• Writing Utensil
• Powers of Observation and curiosity

Instructional Sequence: Part 1 (35-45 minutes)

1. Review what is an observation?
2. Present two identical beakers, labelled #1 and #2. (one contains the alcohol and one contains the water.) Prepared before class.
3. Have students record in their journal as many observations as they can, while you walk around the room with the beakers. Suggest a sketch of the beakers in their journal. (Avoid allowing the students smell the beakers.)
4. When finished writing observations, record class observation on overhead or dry erase board. "There is clear liquid in each beaker (observation)." "There is water in each beaker (inference)."
5. Present two ice cubes with a question. "What do you think will happen if an ice cube is placed in each beaker?" Allow them to predict and record the predictions in their journals. Point out that adding ice is a 'test' for the inference.
6. Add one ice cube to each beaker. Have students write and sketch their observations into journals. Ask them to add as much detail as they can. Circulate around the room with the beakers to give the students a good look at the beakers.
7. What did you observe? (Review first observations.)
8. Why did the ice sink in beaker #1 but not in beaker #2? <<discrepent event. Have students brainstorm and hypothesize. Write hypotheses on the board. (Example of hypothesis: a.#1 contains warm water, #2 contains cold water. b. #2 is water and #1 is some other clear liquid.) You can quickly test these two hypotheses by allowing the students to feel and smell the beakers. They will discover that one clear liquid is alcohol, but...So WHAT? That does not answer WHY the ice sank in alcohol.
9. New question: So, why did the ice sink in the alcohol? (Time to brainstorm again)

Part 2: (25-35 minutes)

1. Tell students that you have two beakers; one contains alcohol and the other contains water.
2. Present two beakers again, but this time, color the water with blue food coloring (in 500 mL beaker) and the alcohol with yellow food coloring (in 200 mL beaker). (Also prepared before class)
3. Loosely drape plastic wrap over the 500mL beaker of water, so that the plastic wrap dips down to the surface of the water.
4. Slowly pour the alcohol onto the plastic wrap.
5. Carefully remove plastic wrap. The yellow-colored alcohol is on top of the blue-colored water. (WOW!)
6. Ask students to predict where an ice cube would 'end up' in this combination of liquid.
7. For Homework: Ask students to write out a prediction about where the ice cube would end up, and explain why they think their prediction is correct. (You can begin next class with reviewing the predictions/explanations, and demonstrate with adding the ice cube to the two-layered combo of liquid.) Perhaps some students will take the initiative to try the experiment at home, or that could be assigned as part of their homework.

Cautions: Teacher should practice combining the colored liquids beforehand. It needs to be done carefully! It would be nice to have small groups perform each experiment for themselves, but in part one, the students would know that the liquids are different due to the alcohol smell; in part two, there is potential for the students to spill/over-mix the liquids. (We want the "WOW" factor)

Investigating the concept of density through observation like this will help the students to really understand this concept, as opposed to simply telling the formula D=m/v. By comparing observations in the first part to the second part, the students should be able to actually SEE that the blue water is heavier than alcohol because they are layered in the beaker. The liquids may have looked the same during the first observation, but clearly they are not the same when compared to the second observation. This opens the door to further investigate/experiment with the density of liquids (such as water, alcohol, salt water, etc.) calculating and graphing those densities, and then the density of solid objects in liquid to predict whether they will sink or float.

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